1. Field of the Invention
The present invention pertains to current limiting fuses in an enclosed environment, such as that provided by enclosed fuse holders installed in an equipment tank.
2. Description of the Related Art
Many types of electrical equipment, especially power distribution equipment are enclosed in tanks which are filled with a heat conducting fluid. When service to interior components of the equipment is required, the tank must be opened and the fluid temporarily removed to permit servicing operations. Current limiting fuses require occasional removal from the equipment tank over the useful life of the electrical equipment which the fuses protect. Fuse canisters have been employed to maintain the current limiting fuses in a dry environment, shielding electrically energized parts of a fuse from the heat conducting fluid filling an equipment tank. Such canisters are widely used in electrical equipment such as underground distribution equipment, including transformers, and switchgear, which are typically filled with either oil or sulphur hexaflouride heat transfer and dielectric media.
It has been found convenient to mount the canisters on an outside wall of the equipment tank, with an open end of the canister accessible from outside the equipment tank, to thereby allow insertion and removal of current limiting fuses and other components therein. Thus, the heat transfer media within the tank need not be disturbed and dismantling of the electrical equipment is not required to service or inspect a fuse within the canister. The open end of the canister is typically enclosed by a locking cap, and the current limiting fuse per se, is mechanically attached to the cap to facilitate its removal using a "hot stick" or other conventional operator equipment.
Although such canisters offer significant advantages in the servicing and operation of a fuse, they do confine the environment surrounding the fuse body, thus giving rise to the possibility that heat transfer away from the fuse body will be significantly reduced. A study performed by J. R. Patel, inventor of the present invention, John S. Schaffer and William R. Rueth, entitled, SF.sub.6 Gas Insulated Load Interruption Switches With Fusing, IEEE Transmission and Distribution Paper 84 T & D 363-8, March, 1984 reported significant heat rises in canister-mounted fuses, suggesting that consideration be given to a possible derating of the current limiting fuses in some circumstances, so as to reduce the heat input into the canister. Of course, it is desirable to overcome such drawbacks, especially if an improvement can be found to eliminate heat buildup with a minimal expense in parts and labor, and without requiring modification to existing equipment.